Narrative:

I had a stay of about 1 hour at ZZZ. Since my first leg had used more fuel than expected due to headwinds; I decided to top off one of my wing tanks for the return flight. I used the self service pump which is new; but rarely used. I did not wait for the fuel to settle; but sumped the fuel immediately; however; I only took out about 15 milliliters. The sample looked fine. After that; I immediately proceeded for run-up and takeoff. Run-up was normal. It is also noted that it had snowed the night before. I noted that the takeoff run was longer than usual; but I attributed this to the altitude. At about 400 ft AGL; the engine started running very rough. I also was barely able to hold altitude and anticipated a possible forced landing straight ahead. I went through the standard emergency procedure; and when I switched tanks and started the boost pump; full power and smooth engine operation immediately returned. The engine continued to operate normally without the boost pump. Later in the flight; over ZZZ1; I switched again to the right tank. This time the engine continued smooth; and the rest of the flight was uneventful. Obviously the partial power loss was due to fuel starvation; it could have been fuel contamination or vapor lock. The aircraft just had an annual about 8 flight hours ago. It is possible that the fuel at ZZZ was contaminated with water; even though I did not find any water during the fuel check. My fuel sample was probably insufficient (15 milliliters) and taken too soon after fueling. If there was water in the fuel; the amount was probably small; since the engine did not stop completely; and later operated normally on the tank in question. In summary; I might have avoided this somewhat scary partial power loss problem if I had waited after topping off the tank to let water settle to the bottom and if I would have taken a larger fuel sample. It is of course possible that the event had another cause than fuel contamination; for example; vapor lock; since the engine was still hot from cruise.

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Original NASA ASRS Text

Title: Bellanca Super Viking suffers partial power loss on takeoff and climb out; possibly the result of water contamination.

Narrative: I had a stay of about 1 hour at ZZZ. Since my first leg had used more fuel than expected due to headwinds; I decided to top off one of my wing tanks for the return flight. I used the self service pump which is new; but rarely used. I did not wait for the fuel to settle; but sumped the fuel immediately; however; I only took out about 15 milliliters. The sample looked fine. After that; I immediately proceeded for run-up and takeoff. Run-up was normal. It is also noted that it had snowed the night before. I noted that the takeoff run was longer than usual; but I attributed this to the altitude. At about 400 FT AGL; the engine started running very rough. I also was barely able to hold altitude and anticipated a possible forced landing straight ahead. I went through the standard emergency procedure; and when I switched tanks and started the boost pump; full power and smooth engine operation immediately returned. The engine continued to operate normally without the boost pump. Later in the flight; over ZZZ1; I switched again to the right tank. This time the engine continued smooth; and the rest of the flight was uneventful. Obviously the partial power loss was due to fuel starvation; it could have been fuel contamination or vapor lock. The aircraft just had an annual about 8 flight hours ago. It is possible that the fuel at ZZZ was contaminated with water; even though I did not find any water during the fuel check. My fuel sample was probably insufficient (15 milliliters) and taken too soon after fueling. If there was water in the fuel; the amount was probably small; since the engine did not stop completely; and later operated normally on the tank in question. In summary; I might have avoided this somewhat scary partial power loss problem if I had waited after topping off the tank to let water settle to the bottom and if I would have taken a larger fuel sample. It is of course possible that the event had another cause than fuel contamination; for example; vapor lock; since the engine was still hot from cruise.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.